High quality flux control system for electron gun evaporation

A high-quality flux control system for electron gun evaporation has been developed and tested for the MBE (molecular beam epitaxy) growth of high-temperature superconductors. The system can be applied to any electron gun without altering the gun itself. Essential elements of the system are a high-bandwidth mass spectrometer, control electronics, and a high-voltage modulator to sweep the electron beam over the melt at high frequencies. The sweep amplitude of the electron beam is used to control the evaporation flux at high frequencies. The feedback loop of the system has a bandwidth of over 100 Hz, which makes it possible to grow superlattices and layered structures in a fast and precisely controlled manner. The drift of the total system is dominated by the temperature drift of the secondary emission multiplier in the mass spectrometer. This drift is typically 1-2%/h for copper as measured by an independent quartz-crystal thickness monitor. The system has been successfully tested with evaporation rates of 0.001 to 1 nm/s for various materials and under ozone and oxygen pressures up to 10^-4 mbar. The feedback system has successfully been used for the fabrication of YBa₂Cu₃O₇ thin films